The herbicidal activity of 2-(4-aryloxyphenoxy)propionic acids and derivatives thereof is well known in the art. Furthermore, optical isomers are often known to exhibit enhanced herbicidal activity over the corresponding racemates. For example, U.S. Pat. No. 4,531,969 discloses that the R-enantiomers of certain 2-(4-aryloxyphenoxy)propionic acids and certain derivatives thereof are distinguished by a considerably enhanced herbicidal action compared to the racemic modifications. Since reduced quantities of herbicide are sufficient to achieve comparable levels of control, the application of mixtures enriched in the more efficacious R-enantiomer offers both economical and environmental advantages.
To exploit the agronomic benefits of these advantages, it is necessary to efficiently resolve racemic mixtures of herbicides that are normally produced industrially. Various methods for obtaining high concentrations of individual enantiomers are known. The most common method of resolution of a racemic modification involves its conversion by an optically active reagent into a mixture of diastereomers which can then be separated on the basis of their different physical properties. Diastereomers are generally separated by fractional crystallization, though occasionally by fractional distillation or by chromatography. Once the diastereomers have been separated, they can be reconverted to the individual enantiomers and the optically active resolving agent can be recovered.
Although the physical separation of diastereomers by crystallization, distillation and chromatography are, for example, well documented, separations based on extraction are relatively unknown.